Power semiconductor devices such as IGBTs (Insulated Gate Bipolar Transistors), BIGTs (Bi-mode Insulated Gate Transistors), MOSFETs (Metal Oxide Semiconductor Field Effect Transistors), IGCTs (Integrated Gate Commutated Thyristors), and the like, are used—among other things—for rectification and inversion of electrical voltages and currents. Usually suitable converters comprise a plurality of power semiconductor switches. Converters are known in different topologies and for different applications for coupling electrical grids with variable-speed drives, for compensating purposes, for energy exchange between two electrical grids, etc. for different power levels.
With continuous development of power semiconductor switches, ever newer converters and circuit topologies are desired and developed for ever higher power and voltage ranges. In the medium and high-voltage range, for example, use is made of converters which have a multiplicity of power semiconductor switches arranged in series and/or parallel to one another or submodules formed thereof in the form of half, full bridges, or the like. Thus, high blocking capability and high current carrying capacity can be provided for the particular application, and the voltages can be increased to the highest levels up to high-voltage direct-current (HVDC) transmission. In the case of the classic 2- and 3-level converters, a higher power density is of interest, which can be achieved by compact design in conjunction with low-loss power semiconductors by means of suitable control and an optimal converter configuration and appropriate cooling.
Semiconductor devices generate in operation essentially conduction and switching losses. Depending on the particular application, the reduction of semiconductor losses is necessary in order to be able to reduce the thermal design requirements or achieve higher system efficiencies. In application fields of grid operation such as, for example, the compensation systems or, for example, the so-called Static Synchronous Compensators (STATCOM), which are pulse-operated converters, which generate a three-phase voltage system with variable voltage amplitude and a phase shift of 90° towards the conduction currents, the HVDC or pumped storage power stations, low converter losses are extremely important. Losses are often assessed here by cost factors in order to account for the corresponding system losses over a period of time (e.g. the service life of a converter) in the system or converter price. In this respect, it is extremely important to reduce losses to a minimum. In addition, it is also advantageous to reduce the losses for rarely occurring operating points with a high load in different applications to a minimum in order to avoid the need for a generally oversized cooling design for these points.
By optimized pulse patterns and system designs it is possible to reduce switching losses and switching frequency to a minimum. Some known methods are aimed at influencing the switching transients when switching on and off power semiconductor switches in order to reduce the switching losses. For example, Laurent Dulau, et al. describes in “A New Gate Driver Integrated Circuit for IGBT Devices with Advanced Protections”, IEEE Transactions on Power Electronics, Vol. 21, No. 1, January 2006, an integrated circuit of a control device (a gate driver) for an IGBT switch, which provides a two-stage switching on and off of the IGBT in order to reduce the reverse current peak from the commutating diode when the IGBT is switched on or to decrease the switch-off overvoltage during shutdown of the IGBT. By increasing the gate voltage to an intermediate level, which is slightly above the IGBT threshold voltage, for a short time before the IGBT is finally turned on, the IGBT collector current and its collector current rise dic/dt can be limited, thus the switching losses can additionally be optimized, taking into account the safe operating area (SOA) of the power semiconductor. Also, by reducing the gate voltage to an intermediate level, which is slightly above the IGBT threshold voltage, for a short time before the IGBT is finally switched off, the collector current drop is limited and thus the possible switch-off overvoltage is reduced, which can also reduce the switch-off losses during the switch-off operation. In applications where a multiplicity of high-frequency power semiconductor switches is switched, for example, in the kHz range, this can substantially reduce the total losses during operation.
U.S. Pat. No. 7,724,065 B2 describes a gate drive circuit, which allows desaturation of an IGBT in the conductive state shortly before it is switched off. As a result, the switching transients can be improved during switch-off, the switch-off losses during desaturation with optimized duration before switch-off can be reduced and, as the circumstances require, the switch-off overvoltage can be reduced.
It has also been proposed to control or regulate the rate of change of the collector current dic/dt and the rate of change of the collector emitter voltage duCE/dt of an IGBT during a changeover procedure in order to minimize switching losses by optimizing the switching transients. Switching losses can be reduced even further by optimizing the gate resistances RGon or RGoff taking into account the limits of the safe operating area of an IGBT and its commutating diode. All these methods are aimed at the optimization of the switch events or transitions.
In addition, it is possible to reduce conduction losses of the semiconductor devices in the conductive state. This can be done, for example, by the physical construction of the component or by appropriate operating conditions. The conduction losses of an IGBT are usually lower during operation with a reduced junction temperature and can be reduced by efficient cooling. However, the latter increases the space required for the components and cooling systems. Power semiconductor switches with low forward voltage, so-called low VCE(sat) transistors, can also be used, but can increase the cost of implementing or entail other disadvantages such as, for example, increased switching losses. There is the desire to reduce the conduction losses in general or depending on the operating point of the converter, because together with the switching losses they account for the total loss of power semiconductor switches and devices formed thereof, e.g. converters.
The required semiconductor surface and size as well as the associated implementation and operating costs are another important aspect relating to the application of power semiconductor switches. For example, modern converters, which are made with IGBTs, are manufactured from the point of view of safe operating area limits of the IGBTs as well as the thermal limits of the IGBTs, which are determined by the transient and static thermal resistances, the load, the electrical operating point and cooling aspects. For certain critical situations, such as short-term overload or surge currents, the thermal load limits of the IGBTs could be exceeded. A safe thermal design of IGBT for these rare events would lead to a costly oversizing of IGBTs, its cooling device and the converter.
From DE 10 2010 006 525 and EP 2 747 260 A2 it is known that semiconductor devices, which can be switched off again, especially IGBTs, can briefly operate outside of the specified parameters, to allow a desaturation for avoidance and high load currents. DE 10 2010 006 525 B4 describes a device for diverting surge currents or transient overvoltages, for example as a result of a lightning strike with a semiconductor switch that can be switched off again, where the switch-on is achieved by applying a static gate emitter voltage outside of the range specified for its gate emitter voltage for continuous operation. Usually, IGBTs in the switched on state are operated at 15 V, whereby according to the specification of the manufacturers the gate emitter voltage for continuous operation should typically not exceed 20 V, because otherwise the insulating oxide layer of the gates will lose much of its service life and as a result it can be destroyed significantly earlier than anticipated, which destroys the IGBT as a consequence. For the short-term or transient surge currents as a result of lightning strikes an increased gate emitter voltage of, for example, 30 V-50 V is proposed. The switching element is switched off again after the short-term or transient surge current has decayed.
EP 2 747 260 A2 describes a three-phase converter with IGBTs as power semiconductor switches, whereby in the event that conditions are detected, which indicate a load short circuit, the gate emitter voltage of at least one power semiconductor switch is increased from a value in the normal operating mode, for example of 15 V, to a higher value. The higher value can be also in a range between about 30 V and about 70 V, outside of the permissible range. With the resulting increase of the branch currents in the fault-free branches, the motor can achieve a balancing of the short-circuit currents and oscillating torques in the motor can be avoided.